An intake-manifold pressure sensor is used, for example, to diagnose the exhaust-gas recirculation in an internal combustion engine and therefore supplies important information for a control unit to control the internal combustion engine. A load signal representing the current load of the internal combustion engine, or the setting of a correct torque selected by the driver, may be derived from the pressure in the intake manifold. In addition, an optimum injection time in an internal combustion engine having throttle control may be ascertained from the load signal.
Because of the high importance of the intake-manifold pressure sensor during the operation of an internal combustion engine, it is desirable for defects in the intake-manifold pressure sensor to be diagnosed early.
Diagnostic methods for detecting the defectiveness of intake-manifold pressure sensors are known in the related art, but only in the case of conventional gasoline engines, in particular spark-ignition engines. The conventional internal combustion engines are distinguished in that their load control is implemented via the throttle valve, where there is a fixed relationship between the pressure in the intake manifold and the load, or between the throttle-valve angle and the load.
An example of such a diagnostic method for conventional spark-ignition engines is described, e.g., in published German patent document DE 199 46 874. Here, three different signals L1, L2, and L3 are produced from different operating parameters, L1 representing the mass flow rate of air that flows into an intake manifold of the spark-ignition engine, L2 representing the pressure in the intake manifold, and L3 representing a fuel signal ascertained from the mass flow rate of fuel. These signals are compared to each other in pairs and united to form combinations when variations occur. Different combinations of deviations are assigned different causes, i.e., different sources of error, for the deviations. Thus, e.g., in a first part of the method, it can initially be deduced that, when a particular deviation is present, either the intake-manifold pressure sensor and/or the exhaust-gas recirculation valve is defective. In a further part of the method, it may then be determined more accurately if the intake-manifold pressure sensor or the exhaust valve is defective. To this end, the pressure in the intake manifold during both the operation of the engine and its stoppage is measured and evaluated during the post-operation of the corresponding engine control unit. If the intake-manifold pressure is the same in both cases, it may be deduced that there is a defect in the intake-manifold pressure sensor; however, if the pressure in the intake manifold while the engine is stopped is less than that when the engine is being operated, it may then be deduced that an exhaust-gas recirculation valve is defective.
In addition, ambient-pressure sensors for use in internal combustion engines are known in the related art. In addition to the intake-manifold pressure sensors, they also supply important information for a control unit to control an internal combustion engine. Ambient-pressure sensors are used for, inter alia, ascertaining the maximum torque of the internal combustion engine. Diagnostic methods for ambient-pressure sensors are also known in the art. In conventional gasoline engines in which the load is controlled via the throttle valve, an ambient-pressure sensor can, however, only be checked for its correct method of functioning, i.e., plausibility-checked, while starting or during full load, since in conventional gasoline engines, a pressure that approaches ambient pressure is only present in the intake manifold under these conditions.
However, in internal combustion engines having variable valve timing, i.e., in internal combustion engines having throttleless load control, the load of the internal combustion engine is no longer controlled via the throttle valve and, therefore, via the pressure in the intake manifold, but rather via a change in its valve timing and/or its valve lift. The internal combustion engines having fully variable valve timing are distinguished by a lower fuel consumption than conventional gasoline engines.
FIG. 3 schematically illustrates such an internal combustion engine 100 having variable valve timing. Internal combustion engine 100 includes an engine block 110 having a piston 112, which moves up and down in it. Connected to the engine block is an intake manifold 120 having a built-in throttle valve 122 and an exhaust pipe 130. However, in contrast to conventional spark-ignition engines, throttle valve 122 is not used for controlling load. The control of the air supply and air exhaust through the intake manifold and the exhaust pipe, and therefore the control of the load of the internal combustion engine, is carried out via valves 140, which are controlled by a control unit 200, the control being implemented with the aid of fully variable timing edges. Instead of a single control unit 200, several control units interconnected by any communication link may also be used for controlling valves 140. The valves may be moved, for example, by electromagnetic or electrohydraulic actuators.
Control unit 200 includes an ambient-pressure sensor 210 for supplying a throttle-valve pressure signal, which represents pressure p_before_DK upstream from throttle valve 122. In this context, ambient-pressure sensor 210 does not directly supply pressure p_before_DK, but it primarily supplies only the ambient pressure, i.e., the air pressure upstream from air filter 150 of the internal combustion engine. Then, actual pressure p_before_DK upstream from the throttle valve may be subsequently derived in either ambient-pressure sensor 210 itself or control unit 200, by subtracting a pressure drop occurring in air filter 150 of the internal combustion engine from the measured ambient pressure.
In throttleless operation of the internal combustion engine, the pressure upstream from the throttle valve must be equal to the pressure in the intake manifold. An intake-manifold pressure sensor 220 is typically provided in the case of the control units of the related art. Intake-manifold pressure sensor 220 provides an intake-manifold pressure signal, which represents pressure p_intake in intake manifold 120 of internal combustion engine 100. Sometimes, they are additionally provided with an ambient-pressure sensor 210.
However, as explained above, since the load control in internal combustion engines having fully variable valve timing is no longer implemented via the throttle valve, all of the already known diagnostic methods for pressure sensors, which are based on deriving a load signal representing the load of the internal combustion engine from the angular position of the throttle valve or the pressure in the intake manifold, are no longer applicable to internal combustion engines having fully variable valve timing.
Therefore, an object of the present invention is to provide a method, a control unit, and a computer program for detecting a defective intake-manifold pressure sensor and/or a defective ambient-pressure sensor in internal combustion engines having fully variable valve timing.